feat(03-06): add Particles visual mode with per-region id/density (Task 3)

src/test/java/com/mythlane/gravityflip/viz/RegionVisualizerTest.java

@@ -75,10 +75,51 @@ class RegionVisualizerTest {
         assertEquals(DebugUtils.FLAG_NO_SOLID, RegionVisualizer.flagsForMode("Outline"));
         assertEquals(DebugUtils.FLAG_NO_WIREFRAME, RegionVisualizer.flagsForMode("Faces"));
         assertEquals(DebugUtils.FLAG_NONE, RegionVisualizer.flagsForMode("Both"));
+        assertEquals(DebugUtils.FLAG_NONE, RegionVisualizer.flagsForMode("Particles"));
         // unknown → Outline
         assertEquals(DebugUtils.FLAG_NO_SOLID, RegionVisualizer.flagsForMode("xxx"));
     }
 
+    @Test
+    void parseMode_particlesAccepted() {
+        assertEquals("Particles", RegionVisualizer.normalizeMode("Particles"));
+    }
+
+    // ---------- Particles branch ----------
+
+    @Test
+    void visualize_particlesMode_callsParticleEmitterOncePerEdgePoint() {
+        List<String> particleCalls = new ArrayList<>();
+        RegionVisualizer.ParticleEmitter pe = (w, id, pos) -> particleCalls.add(id + "@" + pos.x + "," + pos.y + "," + pos.z);
+        RegionVisualizer viz = new RegionVisualizer(
+                t -> fail("errorHandler unexpectedly called: " + t),
+                (w, shape, m, c, o, t, f) -> fail("DebugEmitter should not fire in Particles mode"),
+                pe,
+                (w, r) -> r.run(),
+                () -> 0L);
+
+        // unit box at density=1 → 8 corner points emitted.
+        GravityFlipRegion r = region("pz", "#FFFFFF", "Particles", 1000, 0.5);
+        r.setVisualParticleId("Dust_Sparkles_Fine");
+        r.setVisualParticleDensity(1.0);
+
+        viz.visualize(null, snapshotOf(r));
+        assertEquals(8, particleCalls.size(), "unit box + density=1 → 8 corner emissions");
+        // All calls use the requested id (validation falls open in test context).
+        for (String call : particleCalls) {
+            assertTrue(call.startsWith("Dust_Sparkles_Fine@"), "unexpected call: " + call);
+        }
+    }
+
+    @Test
+    void particleDefaults_absentInConstructedRegion() {
+        // Defaults must match the codec's documented defaults (03-06).
+        GravityFlipRegion r = new GravityFlipRegion("x",
+                new Box(new Vector3d(0, 0, 0), new Vector3d(1, 1, 1)), true);
+        assertEquals("Dust_Sparkles_Fine", r.getVisualParticleId());
+        assertEquals(1.0, r.getVisualParticleDensity(), 1e-9);
+    }
+
     // ---------- matrixFromBox ----------
 
     @Test